JP4683852B2 - Optical isolator - Google Patents

Description

  The present invention relates to an optical isolator that removes return light generated when light emitted from a light source enters various optical elements and optical fibers.

  In an optical communication module or the like, light emitted from a light source such as a laser light source is incident on various optical elements or optical fibers, but a part of the incident light is reflected on the end surfaces or inside of the various optical elements or optical fibers. It is scattered. A part of the reflected or scattered light tries to return to the light source as return light, and an optical isolator is used to prevent the return light.

  Conventionally, this type of optical isolator has a configuration in which a flat Faraday rotator is disposed between two polarizers, and these three components are housed in a cylindrical magnet via a component holder. Normally, the Faraday rotator is adjusted to a thickness that rotates the polarization plane of light having a predetermined wavelength in the saturation magnetic field by 45 degrees, and the transmission polarization directions of the two polarizers are relatively shifted from each other by 45 degrees. The rotation is adjusted as follows.

  The optical isolator having such a configuration requires a Faraday rotator and two polarizers as separate parts and requires a holder for each element. Therefore, the number of parts is increased and the number of assembly steps is increased. Adjustment work is also complicated, which not only increases costs but also makes it difficult to reduce the size. Further, when incorporating the light source module into the light source module, it is necessary to adjust the position of the magnet for forming the saturation magnetic field, that is, to adjust the polarization plane of the optical isolator.

  For this reason, various optical isolators have been proposed in which an optical isolator element of a Faraday rotator and a polarizer and a rectangular magnet are installed on a flat substrate.

For example, Patent Document 1 shows a conventional optical isolator 18 as shown in FIG. 5, and the optical isolator 18 includes optical isolator elements of a Faraday rotator 12 and polarizers 13 and 14 and a rectangular parallelepiped magnet 17. These are arranged on a single flat substrate 16. Here, the polarizers 13 and 14 have a function of absorbing a polarization component in one direction of transmitted light and transmitting a polarization component orthogonal to the polarization component, and the Faraday rotator 12 is a saturated magnetic field. It has a function of rotating the polarization plane of light of a predetermined wavelength by about 45 degrees in intensity. The two polarizers 13 and 14 are cut out so that their respective surfaces in contact with the substrate 16 serve as reference surfaces, and the transmitted polarization directions are 0 degrees and 45 degrees with respect to the reference surfaces.
Japanese Patent Laid-Open No. 13-91899

  However, Patent Document 1 does not specifically describe a method of joining the optical isolator element 11 and the substrate 16 or the magnet 17 and the substrate 16, and each of the Faraday rotator 12 and the polarizers 13 and 14 depends on the joining method. When the optical isolator element 11 and the magnet 17 are arranged on one substrate 16, there are problems that the optical isolator element 11 is dropped, crack bonding strength is lowered, and optical characteristics are deteriorated.

  That is, when the optical isolator element 11 and the magnet 7 are arranged on a single substrate 16, the Faraday rotator 12 needs to be placed while exhibiting an appropriate saturation magnetic field strength. The size of the magnet 7 must be optimally designed and the side surface of the magnet 7 must be placed close to the side surface of each component of the optical isolator element 11. In this case, the magnet 17 is fixed to the substrate 16. The bonding material 15b to be contacted with the optical isolator element 11 and the bonding material 15a for bonding the optical isolator element 11 would be.

  This is because the optical isolator has been miniaturized in recent years with the demand for miniaturization of optical components, and there is a limit to reducing the size in order to obtain an appropriate saturation magnetic field strength of the magnet 7, so that the size is reduced. This is because the distance between the side surfaces of the magnet 7 and the optical isolator element 11 must be shortened.

  Therefore, after the optical isolator element 11 and the magnet 17 are disposed by attaching the bonding material 15 to the substrate 16 and then melted and fixed at a high temperature, and then cooled, the magnet 17 generally heats the members constituting the optical isolator. Since the expansion coefficient is the largest and the ratio of the bonding area is also large, the effect of volume shrinkage is greatly exerted on the adjacent optical isolator element 11, and in particular, the Faraday rotator 12 has a bismuth substitution that is significantly different from the magnet 17. Since the garnet single crystal is used, when the magnet 17 is cooled after bonding at a high temperature, the volume contraction is significant, and stress is applied to the optical isolator element 11, particularly the Faraday rotator, via the bonding material 15 and the substrate 16. The isolator element 11 is dropped from the substrate 16, or the optical isolator element 11 is cracked or the optical isolator 18 There is a case where time may cause degradation of iso configuration properties. Against this background, there has been a demand for a simple and high-performance structure even if it is downsized.

The present invention has been devised in view of these problems. On a substrate, an optical isolator element composed of a flat Faraday rotator and a polarizer, and the optical isolator element and the side faces of each other are close to each other. In the optical isolator, which is arranged and fixed with a magnet that applies a magnetic field to the Faraday rotator via a bonding material, the magnet has a convex portion protruding from a part of a fixed surface of the magnet bonded to the substrate. The convex portion is formed near the second side surface opposite to the side surface of the magnet facing the optical isolator element, and separated from the optical isolator element, and the second It is formed along a pair of end surfaces adjacent to the side surface , and a bent portion between the portion along the pair of end surface sides and the portion near the second side surface is formed in a curved surface. Features and That.

  Furthermore, the optical isolator of the present invention is characterized in that a core substrate approximating a thermal expansion coefficient of the optical isolator element is interposed between the optical isolator element and the substrate.

  According to the configuration of the present invention, since the convex portion, which is partially protruded, is bonded to the substrate on the surface facing the substrate of the magnet, while showing an appropriate saturation magnetic field strength without downsizing the magnet Since the bonding area of the substrate can be reduced, the influence of the stress due to the cooling shrinkage of the magnet on the substrate and the bonding material can be reduced.

  The distance between the side surface of the convex portion formed on the magnet and the side surface of the optical isolator element facing the side surface is longer than the distance between the side surfaces of the magnet and the optical isolator element facing each other. Therefore, since the bonding material for bonding the optical isolator element and the bonding material for bonding the magnet having the highest thermal expansion coefficient among the components of the general optical isolator can be isolated and bonded, It is possible to effectively prevent stress from being applied to the optical isolator element through the material.

  In this case, when the convex portion formed on the fixed surface of the magnet is formed from the second side surface, the respective bonding materials can be completely separated from each other on the substrate, and are remarkably generated by the magnet at the time of bonding. The stress is more difficult to be transmitted to the optical isolator element, particularly the Faraday rotator 2, and good optical characteristics are obtained.

  Further, the convex portions formed on the fixed surface of the magnet are formed on the pair of end surfaces adjacent to the second side surface, so that the stress generated by the magnet at the time of joining is hardly transmitted to the optical isolator element. In addition, the magnets on the substrate can be stably arranged.

  Furthermore, since a core substrate that approximates the thermal expansion coefficient of the optical isolator element is interposed between the optical isolator element and the substrate, the influence of stress can be ignored, thereby further reducing the distance between the optical isolator element and the magnet. In addition to being able to reduce the size, the thermal expansion coefficient of the substrate can be matched to the mounting partner material such as the submount of the semiconductor module, and the range of material selection can be increased.

  As described above, in the present invention, the optical isolator element does not fall off the substrate after bonding even if it is designed to be small, the optical isolator element does not break or chip, and the optical characteristics of the Faraday rotator do not deteriorate. An optical isolator can be manufactured.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 is a perspective view showing an embodiment of an optical isolator according to the present invention.

  As shown in the drawing, the optical isolator 9 is disposed on the substrate 6 so that the side surfaces 10a and 70b of the optical isolator element 1 and the magnet 7 face each other and are close to each other. And 5b.

  The substrate 6 is made of a plate-like body having a rectangular plane, and the material is selected by a mounting method in which the optical isolator 9 is mounted on the semiconductor laser module. For example, when the substrate 6 is fixed to the submount of the semiconductor laser module by YAG welding. , Stainless steel, kovar, permalloy, etc. are selected, and in the case of fixing by soldering, for example, Cr—Pt—Au or Ti—Pt—Au metallization is applied to the mounting surface of the metal or ceramic, glass, etc. The applied material may be used.

  The optical isolator element 1 includes a flat Faraday rotator 2 and flat polarizers 3 and 4. As is well known, in the case of an optical isolator called a one-stage type, the polarizer 3, the Faraday rotator 2, and the polarizer 4 are arranged in the width direction of the substrate 6 in the order of light emission. .

  The transmission polarization direction of the polarizer 3 is set in a direction parallel to one side parallel to the upper surface of the substrate 6 (referred to as a reference side), and the transmission polarization direction of the other polarizer 4 is The direction is set to 45 degrees with respect to the reference side. Here, by fixing the upper surface of the substrate 6 and the reference sides of the polarizer 3 and the polarizer 4 to substantially coincide with each other, the transmission polarization directions of the polarizer 3 and the polarizer 4 are shifted from each other by 45 degrees without adjusting the rotation. When the Faraday rotation angle of the Faraday rotator 2 is approximately 45 degrees, the best insertion loss characteristic and isolation characteristic can be obtained.

  The polarizers 3 and 4 are configured by an absorption polarizer having a function of absorbing a unidirectional polarization component of incident light, or a birefringent polarizer that separates or synthesizes a unidirectional polarization component of incident light. The The absorptive polarizer is made of, for example, polarizing glass having a structure in which ellipsoidal metal particles are dispersed in glass. This polarizing glass is a glass having polarization characteristics by arranging long stretched metal particles in one direction in the glass itself, light having a polarization plane perpendicular to the stretch direction of the metal particles is transmitted, Light with a parallel polarization plane is absorbed. For example, it is made of polarizing glass having a structure in which ellipsoidal metal particles are dispersed in glass. This polarizing glass is a glass having polarization characteristics by arranging long stretched metal particles in one direction in the glass itself, light having a polarization plane perpendicular to the stretch direction of the metal particles is transmitted, Light with a parallel polarization plane is absorbed.

  The Faraday rotator 2 is adjusted to have a thickness at which the polarization direction of light incident at room temperature rotates 45 degrees. Further, when high isolation is required for the optical isolator 9, the rotational deviation between the polarizer 3 and the polarizer 4 is precisely adjusted to 45-α degrees with respect to the polarization rotation angle 45 + α degrees of the Faraday rotator 2. There is a method in which light is incident from the opposite direction (from the side of the polarizer 4), and the polarizer 2 is rotationally adjusted so that the transmitted light is minimized. Therefore, the transmission polarization directions of the polarizer 3 and the polarizer 4 can be cut out by shifting by 45-α degrees in advance, and for example, the transmission polarization direction of the polarizer 4 can be set to 45-α degrees with respect to the reference side. Further, the accuracy ± α of the polarization rotation angle of the Faraday rotator is desirably about 1 degree due to the characteristics of the optical isolator, and it is installed on the upper surface of the substrate 6 with high accuracy.

  The Faraday rotator 2 is, for example, a bismuth-substituted garnet crystal, and the thickness thereof is set so that the polarization plane of incident light having a predetermined wavelength rotates 45 degrees. Generally, in order to rotate the plane of polarization, it is necessary to apply a sufficient magnetic field in the optical axis direction of incident light, and the magnets 7 are arranged on both sides of the side surface of the Faraday rotator 2.

  Here, both surfaces of the Faraday rotator 2, the polarizer 3 and the polarizer 4 are provided with antireflection films for the refractive index n = 1, and the polarizer 3 and the polarizer 4 and the Faraday rotator 2 are in close contact with each other. In this case, an antireflection film for the refractive index n = 1 is applied to the incident / exit surface where the polarizer is in contact with air, and an antireflection film for the counter polarizer is applied to both surfaces of the Faraday rotator.

  The magnet 7 forms a convex portion 7a that protrudes from a part of the fixed surface of the magnet 7, which is a feature of the present invention. The convex portion 7 a is bonded to the substrate 6. Thereby, the junction area with the board | substrate 6 can be made small, showing appropriate saturation magnetic field intensity, without reducing the magnet 7 in size.

  Actually, the surface of the convex portion 7a and the surface of the substrate 6 are bonded via the bonding material 5a. However, the present invention is not limited to this, and the concave portion is formed in the substrate 6 and is joined by fitting with the convex portion 7a. You may make it the structure which does. Thereby, it is possible to maintain sufficient joint strength.

  The convex portion 7 a of the magnet 7 is formed at a position where the distance between the side surface 70 a and the side surface 10 a of the optical isolator element is longer than the distance between the side surface 70 b of the magnet 7 and the side surface 10 a of the optical isolator element 1. Therefore, a marginal space is formed between the convex portion 7a and the optical isolator element 1, and by this space, the bonding material 5a of the magnet 7 and the bonding material 5b of the optical isolator element 1 can be functionally separated and bonded. For this reason, it is possible to effectively prevent stress from being applied to the optical isolator element 1 via the magnet bonding material 7 during curing.

  Furthermore, as shown in FIG. 1, the convex part 7a formed in the magnet 7 is formed from the side surface 70c of the fixed surface. This also makes it possible to almost completely prevent stress from being applied to the optical isolator element 1, particularly the Faraday rotator 2, as described above.

  The height h of the convex portion 7a of the magnet 7 (described with reference to FIG. 2A) is 0.05 mm to 0.3 mm because the thickness of the bonding material 5a is 0.03 mm to 0.1 mm depending on the bonding material. Any is suitable. This is because if the height h is made larger than this, the magnetic field strength applied to the Faraday rotator 2 may be insufficient.

  As a material of the magnet 7, for example, a material made of a samarium cobalt sintered magnet is suitable. The magnets 7 are disposed on both sides of the optical isolator element 1, and the magnetic poles are disposed on the magnet 7 in such a direction that the magnetic field lines passing through the Faraday rotator 2 are maximized. The rotator 2 has a magnetic field intensity sufficient to rotate the polarization plane of incident light having a predetermined wavelength by 45 degrees. The shape of the magnet 7 is not limited to this, and may be one as long as the Faraday rotator satisfies a predetermined magnetic field strength, and the shape other than the contact surface with the substrate 6 is not limited.

  The magnet 7 is sintered after pressing the material powder, and the surface is usually plated with Ni in order to improve the corrosion resistance and impact resistance. In the case of a samarium-cobalt sintered magnet, it has excellent corrosion resistance, but Ni plating is appropriately performed depending on the bonding material.

Thus, according to the configuration of the present invention, the thermal expansion coefficient of the Faraday rotator 2 is generally about 10 × 10 ⁻⁶ / ° C. because the bismuth-substituted garnet is used, and the polarizers 3 and 4 are generally polar cores ( The product has a coefficient of thermal expansion of about 6.5 × 10 ⁻⁶ / ° C., and the magnet 7 generally uses a sintered samarium cobalt magnet. ^-6 / ° C. Even if these members having different coefficients of thermal expansion are firmly mounted on the single substrate 6, the substrate 6 is bonded to the contact surface of the magnet 7 only by the convex portion 7 a, so that the bonding material 5 a is bonded on the substrate 6. The material 5b does not come into contact with each other, and stress generated remarkably by the magnet 7 becomes difficult to be transmitted to the optical isolator element 1, particularly the Faraday rotator 2, so that good optical characteristics can be obtained.

  FIG. 2 is a sectional view for explaining the effect of the present invention.

  FIG. 2A is a plan view of the optical isolator, showing the position of the AA cross section passing through the magnet 7 and the Faraday rotator 2. FIG. 2B is a vertical cross-sectional view of the conventional optical isolator showing a shrinkage behavior at the time of joining at the position of the AA cross section. FIG.2 (c) is a longitudinal cross-sectional view at the time of cut | disconnecting the state of the member in the optical isolator of this invention in the position of the above-mentioned AA cross section.

  FIG. 2A shows a state in which the bonding material 5 is placed on the entire upper surface of the substrate 6 and the magnet 7 and the Faraday rotator 2 are fixed by the bonding material 5. FIG. 2B shows a state in which the bonding materials 5a and 5b are formed in different regions on the substrate 6 without contacting each other, the magnet 7 is fixed by the bonding material 5a, and the optical isolator element 1 is fixed by the bonding material 5b. Show.

  The object expands with increasing temperature and contracts with decreasing temperature. The amount of shrinkage per unit length caused by a change in temperature of 1 ° C. is called a thermal expansion coefficient, and this is represented by α. The stress at each joint begins to occur near the melting point of the joining material, and the residual stress increases as the temperature decreases from here. The arrows in the magnet 7 and the Faraday rotator 2 in the figure indicate the direction in which each member contracts when the temperature drops.

  Further, it has been found that the extinction ratio of the Faraday rotator 2 of the optical isolator element 1 is greatly reduced by the residual stress, particularly the tensile stress. The extinction ratio of the Faraday rotator 2 is a ratio of power indicating how much of the incident linearly polarized light rotates while maintaining the linearly polarized light.

  From the above, in the configuration of FIG. 2A, the magnet 7 having a large coefficient of thermal expansion contracts greatly with the temperature drop from the melting point temperature of the bonding material to room temperature, and the bonding material 5 is interposed in the direction of arrow F in the figure. Since the Faraday rotator 2 is pulled, problems such as a decrease in the extinction ratio of the Faraday rotator and cracks in the bonding material 5 arise. On the other hand, in FIG. 2B of the present invention, since the bonding materials 5a and 5b are separated, the Faraday rotator is not easily affected by the contraction of the magnet 7, and the characteristics of the Faraday rotator are deteriorated. And cracks in the bonding material are less likely to occur.

  FIG. 3 is a perspective view showing a second embodiment of the optical isolator of the present invention.

  The optical isolator 10 of this embodiment is different from the optical isolator 9 of FIG. 1 in that a core substrate 8 is interposed between the substrate 6 and the optical isolator element 1. For example, under the conditions using alumina ceramics as the substrate 6, bismuth-substituted garnet single crystal as the Faraday rotator, cupo (product name of HOYA Corporation) as the polarizer, and samarium cobalt magnet as the magnet, It is appropriate to use, as the core substrate 8, zirconia ceramics whose member has a thermal expansion coefficient closest to that of the garnet crystal.

  According to this configuration, by selecting a thermal expansion coefficient of the core substrate 8 close to that of the Faraday rotator 2, the influence of stress can be ignored, so that the distance between the optical isolator element 1 and the magnet 7 is further reduced. In addition to being able to reduce the size, the coefficient of thermal expansion of the substrate 6 can be matched to the material of the mounting partner such as the submount of the semiconductor module, which increases the range of material selection.

  This embodiment also has the same effect as the first embodiment, and is configured such that the optical isolator element 1 is more difficult to receive tensile stress from the magnet 7.

  FIG. 4 is a view showing a third embodiment of the present invention, in which (a) is a plan view and (b) is a partially enlarged plan view of (a). The feature of the present embodiment is that the protrusion 7a is formed in a U shape or an inverted U shape so as to surround the optical isolator element 1. That is, the convex portion 7a is formed on the fixed surface of the magnet 7 from the side surface 70c opposite to the side surface 70b of the magnet 7 facing the optical isolator element 1, and the pair of the magnets 7 adjacent to the side surface 70c. Is formed on the end face 70d side.

  As a result, not only can the stress be prevented from being applied to the Faraday rotator of the optical isolator element 1, but even if the magnet 7 is arranged on the substrate 6, the Faraday can be reduced while reducing the bonding area between the magnet 7 and the substrate 6. Since the distance to the rotor 2 can be increased, the magnet 7 is stably disposed with respect to the substrate 6, and the stress generated in the isolator element 1 can be reduced. In this case, as shown in (b), the bent portion of the protruding portion 7a is formed in R. As a result, it is possible to reduce stress that tends to concentrate on the bent portion of the magnet 7 itself.

  As an example of the present invention, the optical isolator shown in FIG. 1 was prototyped and subjected to characteristic evaluation and reliability test. Each component and configuration will be described below.

A polar core (product name) manufactured by Corning Inc. was used as the polarizer, and the size was 10 mm square and the thickness was 0.2 mm. . A thermal expansion coefficient of 6.34 × 10 ⁻⁶ (1 / ° C.) at this time was used.

As the Faraday rotator, a bismuth-substituted garnet was used, and a 10 mm square and 0.4 mm thick one was cut into a 1 mm square. In this case, the polarization rotation angle in the saturation magnetic field strength was 45 degrees. The thermal expansion coefficient at this time was 10.0 × 10 ⁻⁶ (1 / ° C.).

  Both the polarizer and the Faraday rotator are elements that operate with respect to light having a wavelength of 1.55 μm, and antireflection films of air (n = 1) are provided on both surfaces of the polarizer and the Faraday rotator. In addition, a Ti—Pt—Au multilayer metal film was formed on one side serving as a bonding surface between the polarizer and the Faraday rotator.

Zirconia ceramics was used as the substrate material, and a Ti—Pt—Au multilayer metal film was formed on the surface in advance. The thermal expansion coefficient of the zirconia ceramics is 10.5 × 10 ⁻⁶ / ° C., which is almost the same as the thermal expansion coefficient of the Faraday rotator, and the Faraday rotator is not affected by the stress from the substrate.

  The size of the substrate was a width W = 3 mm, a length D = 1.5 mm, and a thickness t = 0.3 mm. In addition, a bonding material made of Au / 20Sn foil brazing material having a width W = 0.8 mm and a length D = 1.2 mm is placed in a substantially central part on the substrate, and a width W = 0.7 mm on both sides of the bonding material. The same bonding material made of Au / 20Sn foil brazing material having a length D = 1.2 mm was placed. The two bonding materials were separated by a width of about 0.5 mm.

The magnet was made of samarium cobalt, and two magnets formed in a substantially rectangular parallelepiped having a width W = 0.8 mm, a length D = 1.4 mm, and a thickness t = 1.4 mm were used. The thickness of the brazing material was selected to be 0.03 mm, and the step height of the magnet joining surface was set to 0.2 mm. The thermal expansion coefficient at this time was 6.5 × 10 ⁻⁶ (1 / ° C.) in the optical axis direction and 13.0 × 10 ⁻⁶ (1 / ° C.) in the direction perpendicular to the optical axis.

  Next, as shown in FIG. 1, the optical isolator element is arranged on the substrate, and the magnet is placed on the substrate and heated and melted at 280 ° C. in a nitrogen atmosphere with 3 g pressed, A magnet was fixed to the substrate.

  In this manner, 50 optical isolators were manufactured and the characteristics were measured. As a result, it was confirmed that all the optical isolators had good and uniform characteristics with an insertion loss of 0.3 dB or less and an isolation of 35 dB or more. While the average value of the isolation characteristic was as low as 23 dB when the prototype was manufactured with the conventional configuration, the average value was as high as 42 dB with the configuration according to the present invention, and good results were obtained.

  Next, the reliability of the produced optical isolator was evaluated.

  The tests were conducted vibration test, impact test, temperature cycle test, high temperature holding test, low temperature holding test, high temperature high humidity test shown in Telcordia 1221. In all tests, the amount of change in insertion loss is ± 0.2 dB or less, A good result was obtained in which the amount of change in isolation was ± 3 dB or less.

  By the above trial manufacture, there is little influence of stress due to the volume shrinkage of the magnet, the optical characteristics are stable, the assembly is easy and the man-hours are small, and the optical isolator element does not fall off, cracks, and has excellent reliability. An optical isolator can be provided.

It is a figure which shows Embodiment 1 of the optical isolator of this invention. (A) is a top view of an optical isolator, (b) is a longitudinal cross-sectional view when the figure which shows the shrinkage | contraction behavior at the time of joining in the conventional optical isolator is cut in the position of the AA cross section of (a). . (C) is a longitudinal cross-sectional view at the time of cut | disconnecting the state of the member in the optical isolator of this invention in the position of the AA cross section of (a). It is a figure which shows Embodiment 2 of the optical isolator of this invention. It is a figure which shows Embodiment 3 of the optical isolator of this invention, (a) is a top view, (b) is a partially expanded plan view of (a). It is a figure which shows the structure of the conventional optical isolator.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 11 Optical isolator element 2, 12 Faraday rotator 3, 4, 13, 14 Polarizer 5, 15 Bonding material 5a, 5b, 5c Bonding material 6, 16 Substrate 7, 17 Magnet 7a Protruding part 8 Core substrate 9, 10 , 18 Optical isolator h Step height

Claims (2)

An optical isolator element composed of a flat-plate Faraday rotator and a polarizer, and a magnet for giving a magnetic field to the Faraday rotator and a side surface of the optical isolator element facing each other in close proximity to each other In the optical isolator that is fixed via, the magnet has a convex portion that protrudes from a part of the fixing surface of the magnet and is fixed to the substrate, and the convex portion is opposed to the optical isolator element. The second side surface opposite to the side surface of the magnet is formed apart from the optical isolator element, and is formed along a pair of end surface sides adjacent to the second side surface , An optical isolator , wherein a bent portion between a portion along the pair of end surfaces and a portion near the second side surface is formed into a curved surface . The optical isolator according to claim 1, wherein a core substrate approximating a thermal expansion coefficient of the optical isolator element is interposed between the optical isolator element and the substrate.

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